This invention relates to the recovery of hydrocarbons from oil-bearing rock composed primarily of limestone and is particularly concerned with an above-ground extraction process which permits the recovery of these hydrocarbons in substantial quantities.
A large amount of oil exists today in the United States trapped in deposits of limestone located throughout the country. The oil normally exists in the form of an asphalt that is solid at ambient conditions. The current shortage of oil has made it highly desirable to recover liquid hydrocarbons from these heavy oil-bearing deposits. It has been suggested that conventional methods of in situ steam stimulation used in the past with success in recovering oil from tight formations of sand be applied in an attempt to recover heavy oil from limestone deposits. Such methods normally include drilling a series of several boreholes into the formation around a central borehole and injecting high pressure steam into the central borehole. The heat from the steam moves by conduction and convection outward from the central borehole decreasing the viscosity of the trapped oil and forcing it toward the other boreholes from which it is eventually recovered. Attempts to apply such methods for recovering the oil from limestone deposits in southwest Texas, however, have proven ineffective evidently because the viscosity of the oil is so great and the permeability of the formation so low that it is impossible to force the oil or steam through the limestone.
In addition to attempting to recover the oil by in situ steam stimulation, it has been suggested that the heavy oil-bearing limestone be mined and then subjected to pyrolysis in an above-ground retort thereby recovering the oil in a process similar to that used to recover liquid hydrocarbons from oil shale. Such pyrolysis processes normally involve heating the oil-bearing limestone to a temperature between about 700.degree. F. and 900.degree. F. in order to crack and volatilize the oil thereby forcing it out of the pores of the limestone. Although such a process works effectively, it has some major disadvantages. The primary disadvantage is that the process involves the use of substantial amounts of energy in order to heat the large volume of limestone rock that must be passed through the retort in order to produce a significant yield of liquids.